I think that so much of this is down to driving style. In my own experience of driving a TwinAir 4x4, I have found that the engine is quite capable of producing averages of 50mpg+, but it's also very easy to drop that down to 40-or-so-mpg if being less cautious.
I firmly believe that the "Eco" button does help, not so much by changing the parameters, but by delivering a more graduated throttle response, allowing more precise modulation of the throttle to maintain speed or gently accelerate. That's why some get similar mpg with it turned off - you can achieve the same result by being feather-light on the throttle.
The TwinAir is deceptive in it's delivery - even in Eco-mode, I've found it's possible to keep up with the flow of traffic on small throttle openings, delivering much better mpg. It just delivers that in a deceptive way. And this is where I think many go 'wrong' - because the 85/90bhp performance is there, along with corresponding torque at low revs and moderate-large throttle openings, coupled to having the Eco-function switched off, we use more fuel than strictly necessary. I wouldn't be surprised if the TwinAir delivers performance not dissimilar to the 1.2 4-cyl engine, whilst the TwinAir is in Eco, with corresponding mpg, but it feels slower because of that deceptive delivery?
The best I've had over ~180miles was 58mpg over mostly motorway, sitting at 50mph through roadworks and 60-70mph on open motorway. That's not far off Fiat's claims.... Remember that all cars nowadays are tuned to deliver under test conditions which involves acceleration cycles probably slower than those typically used day-to-day on the road. Also, "extra-urban" on those tests is a steady 56mph, not the 70+mph that most are trying to maintain and expect that same economy.
